He was a major star at Metro-Goldwyn-Mayer during and after World War II, known for his upbeat and "all-American" screen persona, often playing young military servicemen,[3] or in musicals.

Originally a Broadway dancer, Johnson achieved his breakthrough playing a rookie bomber pilot in A Guy Named Joe (1943).

Throughout the war years, he became a popular Hollywood star, as the embodiment of the "boy-next-door wholesomeness" playing "the red-haired, freckle-faced soldier, sailor, or bomber pilot who used to live down the street"[3] in such films as The Human Comedy (also 1943) and Thirty Seconds Over Tokyo (1944).

[3] After World War II, he continued to play similar heartthrob and military characters, equal parts in serious dramas like The Caine Mutiny (1954), and in light musicals like Brigadoon (1954).

[5] Johnson toured New England in a theater troupe as a substitute dancer, but his acting career began in earnest in the Broadway revue New Faces of 1936.

[7] In 1939, director and playwright George Abbott cast him in Rodgers and Hart's Too Many Girls in the role of a college boy and as understudy for all three male leads.

He had an uncredited role in the film adaptation of Too Many Girls, which costarred Lucille Ball and Desi Arnaz, then Abbott hired him as a chorus boy and Gene Kelly's understudy in Pal Joey.

[8] Johnson was about to move back to New York when Lucille Ball took him to Chasen's Restaurant, where she introduced him to MGM casting director Billy Grady who was sitting at the next table.

[9] Johnson's all-American good looks and easy demeanor were ill-suited to the gritty movies that Warner made at the time, and the studio dropped him at the expiration of his six-month contract.

He attracted attention in a small part in The War Against Mrs. Hadley (1942), and this encouraged MGM to cast him in their long-running series Dr. Kildare.

During the film's production in 1943, Johnson was involved in a serious car accident that left him with a metal plate in his forehead and a number of scars on his face that plastic surgery could not completely correct or conceal.

MGM wanted to replace him in A Guy Named Joe, but Tracy insisted that he be allowed to finish the picture, despite his long absence.

He played an officer leading Japanese-American troops of the famed 442nd Regimental Combat Team in Europe in the Schary-produced film Go for Broke!

One commentator noted years later that "Humphrey Bogart and Jose Ferrer chomp up all the scenery in this maritime courtroom drama, but it's Johnson's character, the painfully ambivalent, not-too-bright Lieutenant Steve Maryk, who binds the whole movie together."

Featuring Claude Rains in his only singing and dancing role, it aired on November 26, 1957, as part of NBC's week of Thanksgiving specials.

On February 19, 1959, Johnson appeared in the episode "Deadfall" of CBS's Dick Powell's Zane Grey Theatre in the role of Frank Gilette, a former outlaw falsely charged with bank robbery.

He is framed by Hugh Perry, a corrupt prosecutor played by Harry Townes, and Deputy Stover, portrayed by Bing Russell.

Convicted of the robbery, Gilette is captured by outlaws while on his way to prison, and the sheriff, Roy Lamont, portrayed by Grant Withers, is killed.

In 1961 Johnson traveled to England to star in Harold Fielding's production of The Music Man at the Adelphi Theatre in London.

The show enjoyed a successful run of almost a year, with Johnson playing the arduous leading role of Harold Hill to great acclaim.

He also appeared in a special two-part episode of The Love Boat, "The Musical: My Ex-Mom; The Show Must Go On; The Pest, Parts 1 and 2" which aired on February 27, 1982, and co-starred Ann Miller, Ethel Merman, Della Reese, Carol Channing, and Cab Calloway.

In 1985, returning to Broadway for the first time since Pal Joey, he was cast in the starring role of the musical La Cage aux Folles.

Johnson married former stage actress Eve Abbott (1914–2004) on January 25, 1947, the day after her divorce was finalized from actor Keenan Wynn.

In a statement by Eve, published after her death at age 90, she said MGM had engineered her marriage to Johnson to cover up his alleged homosexuality.

[22] Johnson's marriage to Eve Abbott ended four years after Mayer's death when Johnson, performing as Professor Harold Hill in The Music Man in the West End in London, is alleged to have begun an affair with a male dancer in the production, according to her son Ned Wynn.

[23][24] In contrast to his "cheery Van" screen image, Eve claimed that he was morose and moody because of his difficult early life.

Reflecting on his career after his death, one critic observed that Johnson was "capable of an Oscar-worthy performance, and that's more than most movie stars can claim".